Liquid crystal displays (LCDs) are increasingly being used for the display device in televisions, personal computers, etc., and in much state-of-the-art equipment such as automotive navigation systems and simulation devices. Many LCDs use what is referred to as a fixed color palette. One of the problems associated with using a fixed color palette is that many of the colors needed to provide a satisfying image may not be available in the palette. In order to overcome this problem dithering is used to create the illusion of color depth in images with a limited color palette (color quantization). More specifically, dithering is an intentionally applied form of noise, used to randomize quantization error, thereby preventing large-scale patterns such as “banding” (stepwise rendering of smooth gradations in brightness or hue) in images. In a dithered image, colors not available in the palette are approximated by a diffusion of colored pixels from within the available palette. The human eye perceives the diffusion as a mixture of the colors within it. In this way, dithering takes advantage of the human eye's tendency to “mix” two colors in close proximity to one another. Typically, in most video displays, dithering is performed \using a fixed pattern that has been optimized for the particular video display. For example, for every pixel in the image the value of the pattern at the corresponding location is used as a threshold. Therefore, since different patterns can generate completely different dithering effects, there is generally a well defined dithering pattern used for a particular display device. The conventional technique of dithering is utilized to display many colors and grey scales on a display device having relatively few colors and grey scales without having to change the resolution of the display device. For example, through the use of the dithering technique, it is possible to display a 16-color image on a display device having only an 8-color palette. Similarly, by using dithering, it is possible to display an image formed from 16 grey scales on a binary display device in which each pixel can only be turned on or off. The underlying principle of dithering is to rely on a particular spatial distribution of illuminated pixels and non-illuminated pixels to reproduce the color and/or brightness of an original image on the display.
It is well known that every pixel in an LCD must be driven with an AC signal in order to avoid permanent damage to the liquid crystal since the liquid crystal material degrades if the electric field is applied to the liquid crystal material continuously in the same direction. Therefore, in order to avoid damaging the liquid crystal, the AC drive signal causes the direction in which the electric field is applied to be constantly changed where the switching of electrode voltage values between positive and negative values is referred to as inversion drive. Unfortunately, however, what is referred to as a kickback voltage is generated by parasitic capacitance in the pixels such that the RMS of the positive voltage is different from the RMS of the negative voltage. Accordingly, the amount of light permeating the liquid crystal material in the odd frames and that of light permeating the liquid crystal material in the even frames is different resulting in what is commonly referred to as screen (or luminance) flicker observed in units of one-half of frame frequency of, for instance, 60 Hz (or 30 Hz).
A common approach to avoiding this flicker, in every frame only approximately one half of the pixels are driven with a positive voltage whereas the remaining pixels are driven with a negative voltage. Typically the positively driven and negatively driven pixels are interleaved in what is referred to as a pixel inversion pattern in order to further mitigate any flicker. Such pixel inversion patterns can take many forms, such as a checkerboard pattern, etc. However, in some instances, the pixel inversion pattern can interact with a dither pattern to create unacceptable video artifacts. For example, the pixel inversion pattern and the dither pattern can interact in such a way as to create visible banding in the display video image.
Therefore, it would be desirable to provide system that harmonizes a pixel inversion pattern and a dither pattern for a display.